I have seen both this post and your initial post and to be honest, this is really a trick question.

You are correct in that you could install a Tee extending the existing cleanout outwards with a new cleanout cap on the end and tie in a horizontal branch drain on the side inlet, however you may not tie in an unvented waste arm from the washer standpipe.

The problem here is that there is a toilet connected to the vertical stack on the floor above. We may not connect a waste arm into a vertical stack below a toilet.

This now leaves us with a couple options.

1. You could cut into the concrete and make a tie in to the horizontal drain by means of a wye & 1/8 bend, then run a horizontal branch over and up to your washer standpipe location. In this configuration the washer standpipe would vent directly from the main drain.

2. A second option would be to tie a horizontal line into the vertical stack at the cleanout in the manner you describe, then move horizontal a short distance and establish a vent. The section of line from the vent to the vertical stack would then be classified as a horizontal branch and it may connect to the stack. A short waste arm could then continue horizontal to your laundry standpipe trap and the standpipe could then turn vertical, providing thier is a minimum of 18" vertical standpipe above the water level in the trap.

If you happen to be in an area which is under the International Residential Code you could create the auxillary vent by means of an air inlet valve.

If you are in an area under the Uniform Plumbing Code, and if creating a vent through the roof would not be practical, you may be permitted to create the vent by means of an air inlet valve with the expressed permission of your local code inspection authority (The UPC permits a maximum of one AAV per structure.) Another option which may be permitted by your local inspector would be to create and "Island Loop Vent" at the point where you are installing the standpipe.

A person can get hasty, trying to help people out when you see someone asking for help and there are no reply posts up on the screen.

You are refering most likely to one of those Studer-type vents in your last paragraph? We have had new bathrooms put in by a master plumber who have put these in when installing bathrooms in new locations, away from the original vent. They may have pulled a "sneaker", but they work, and I have never smelled any sewer gas smells in the places we have them at.

air inlet valve

Lazypup,
After studying the situation it seems like option #2 would be the best option - especially if I don't want to dig up concrete. Regarding the air inlet valve...Is there any difference between the "name-brand" valve thats about $28 versus the simple one(made by a leading plumbing supplier) for $7?
I have studying your drawings in other posts great!
Thanks!